The discovery and development of organic fluorophores is essential for progress in many areas of chemistry, biology and functional materials research. As an emerging subclass, fluorescent organic salts are gaining much attention because of their unique properties. Because of their charged nature, they impart high thermal stabilities, phase tunabilities, water-solubility and chemoselective sensing via electrostatic interactions. These unique features have inspired research in a number of areas including fundamental photophysical investigations, sensory materials, and novel materials for display applications, multiphoton excitation (MPE), nanoscopic fluorescent ionic liquids, and LEC cells. In recent years, the highly photostable, less susceptible to environmental change and high specificity bearing a good photoactivatable fluoro-phore are powerful tools in biochemical and biological research such as cell lineage in development, macromolecule tracking in living cells, and super-resolved fluorescence imaging such as photoactivated localization microscopy (PALM), stochastic optical recon-struction microscopy (STORM), etc. Tracking cell organelles like the dynamics of mitochondrial morphology has attracted much research interest because of its involvement in early stage apoptosis and degenerative conditions. Therefore, developing a modular approach to novel organic emissive salts with tunable photophysical properties are highly warranted.
Over the last decade, homogeneous gold catalysis has emerged as a powerful tool for building molecular complexity in an atom-economical fashion. Gold catalysts act as π-acid and hence they are capable of activating unsaturated carbon-carbon bonds for the addition of nucleophiles (Scheme 1, path a). While manifestation of gold-catalyzed reactions are likely to be continued in the future, there is an urgent need to enable new modes of reactivities to expand gold-alkyne catalysis toolbox. In recent years, oxidative gold-catalyzed reactions have emerged as a new research field (path b). In 2007, Toste and Zhang independently reported gold(I)-catalyzed oxidative rearrangements of alkynyl sulfoxides which proceed via α-carbonyl gold-carbenoids formed through oxygen atom transfer from the sulfoxide. On the basis of this reactivity, the research groups of Liu, Davies, Ye, Zhang and others reported a variety of oxidative gold catalyzed reactions.
Yet another approach in the oxidative gold catalyzed reaction feature external oxidant-poared Au(I)/Au(III) catalysis, where the metal oxidation state changes during the catalytic cycle.
A historical retrospect on the progress in the field of external oxidant-poared Au(I)/Au(III) catalysis reveals that the chemistry is either based on oxidative dimerization reactions of insitu generated vinylgold species (Scheme 1, eq 1) or cross-coupling reactions between respective substrates (eq 2). In recent years, gold-catalyzed aminoarylation/oxyarylation reaction of alkenes and arylboronic acids is emerging as new technique for heterocyclic synthesis (eq 3). Pioneering work from the group of Zhang and Muñiz reported the amination-arylation and diamination of alkenes via Au(I)/Au(III) catalysis (eq 4).
Article titled “Homogeneous Gold-Catalyzed oxidative carboheterofunctionalization of alkenes” by Guozhu Zhang et al. published in Journal of American Chemical Society, 2010, 132 (5), pp 1474-1475 reports Homogeneous carboamination, carboalkoxylation and carbolactonization of terminal alkenes are realized via oxidative gold catalysis, providing expedient access to various substituted N- or O-heterocycles. Deuterium-labeling studies established the antinature of the alkene functionalization and the indispensible role of Au(I)/Au(III) catalysis. This study constitutes the first example of catalytically converting C(sp3)-Au bonds into C(sp3)-—C(sp2) bonds in a cross-coupling manner and opens new opportunities to study gold alkene catalysis where alkylgold intermediates can be readily functionalized intermolecularly.
Article titled “Au(I)/Au(III)-catalyzed Sonogashira-type reactions of functionalized terminal alkynes with arylboronic acids under mild conditions” by Deyun Qian et al. published in Beilstein Journal of Organic Chemistry, 2011; 7: 808-812 reports a straightforward, efficient, and reliable redox catalyst system for the Au(I)/Au(III)-catalyzed Sonogashira cross-coupling reaction of functionalized terminal alkynes with arylboronic acids under mild conditions has been developed.
Article titled “An efficient and recyclable magnetic-nanoparticle-supported Palladium catalyst for the Suzuki Coupling reactions of Organoboronic acids with Alkynyl Bromides” by Xiuli Zhang et al. published in Synthesis, 2011, 2975-2983 reports a highly active, air- and moisture-stable and easily recoverable magnetic-nanoparticle-supported palladium catalyst enables the Suzuki cross-coupling reaction of alkynyl bromides with organoboron derivatives in very good yields in ethanol. The supported palladium catalyst can be recovered and reused up to 16 times without significant loss of catalytic activity.
Article titled “Gold-catalyzed intramolecular aminoarylation of alkenes: C—C bond formation through bimolecular reductive elimination.” by Brenzovich W E Jr et al. published in Angewandte Chemie International Edition, 2010 Jul. 26; 49(32):5519-22 reports Gold-catalyzed intramolecular aminoarylation of alkenes. Article titled “Gold-catalyzed carbon-heteroatom bond-forming reactions” by A. Corma et al. published in Chemical Reviews, 2011, 111, 1657-1712 reports gold-catalyzed transformations involving any carbon-heteroatom bond formation.
There exists no report on the di-functionalization of alkynes utilising Au(I)/Au(III) catalysis. There is need to develop tunable molecules for mitochondrial imaging and to develop intramolecular 1,2-aminooxygenation of alkynes to access pyridinium-oxazole dyad salts.